Polymerization processes for producing polymers, such as polyolefins, typically require removal of unreacted monomers and solvents (i.e., volatile components) from final product. The monomers and solvents are subsequently recycled back into the polymerization process. Usually, removal of the monomers and solvents is achieved through devolatilization of the polymer solution stream. Devolatilization may be achieved by raising the temperature of the polymer solution, e.g., above the boiling point of the polymer solution, and removing the volatile components. The partial pressure of the volatile components may also be lowered. For example, a polymer solution stream may be passed through a preheater and then introduced into a devolatilizer, such as a chamber or vessel, with reduced pressure.
Typically, shell and tube heat exchangers and plate heat exchangers have been used as the preheater during devolatilization. A shell and tube heat exchanger (STHE) consists of a shell containing a bundle of tubes where the polymer solution may flow through the tubes (tube side) and a heating fluid may flow outside the tubes but inside the shell (shell side) to transfer heat between the polymer solution and the heating fluid. For example, EP 0359432 generally discloses a STHE used as a preheater, which is connected to a vacuum vessel with a distributor to reshape the polymer strands. US EP0359432 reports a STHE connected to a devolatilizer through a plate-like distributor for devolatilizing a polymer solution with high solid content. Additionally, U.S. Pat. No. 4,954,303 discloses a STHE mounted on top of a vacuum chamber with a low shear rate mixer for increasing volatile vaporization surface area.
A plate heat exchanger (PHE) consists of multiple heated flat plates arranged in layers having channels connecting an interior portion where a polymer solution is introduced and an exterior portion where the polymer solution may be heated and devolatilized. For example, U.S. Pat. No. 5,453,158 reports a PHE with varying channel width, which is embedded in a closed shell to devolatilize a polymer solution and U.S. Pat. No. 4,808,262 discloses a method for devolatilizing polymer solutions by heating the polymer solutions.
Although STHE and PHE have been used as preheaters during devolatilization of polymer solution for many years, there still exists many problems with their use. In particular, polymer solution undergoing devolatilization usually has a high solid content (e.g., about 15 wt % to about 90 wt %) rendering it a highly viscous laminar fluid. Thus, a large heat transfer area is required to meet the necessary heat transfer efficiency. With respect to STHE, a large heat transfer area translates into an increase in tube number and shell size resulting in increased equipment size and cost. Moreover, such an increase in tube number and shell size can lead to an undesirably high pressure drop as polymer solution flows through the tubes; thus, requiring more expensive pumps. Furthermore, where tube diameter is large, a high radial temperature gradient can exist. For example, a maximum radial temperature difference can be as high as 300° F. (149° C.) when a 0.75 inch tube diameter is used. A high radial temperature gradient where there is a high temperature close to the tube wall can cause degradation of polymer while low temperature close to the center of the interior of the tube can hinder devolatilization. As discussed in Green, D. W., Perry, R. H. (Editors) Perry's Chemical Engineers' Handbook (8th Edition). Chapter 11: Heat Transfer Equipment. McGraw-Hill, New York, 2007, while decreasing tube diameter can improve issues with non-uniform heating, pressure drop is greatly increased.
Additionally, polymer fouling may occur in the preheaters. Polymer may foul and accumulate in the tube bundles in a STHE, especially when the tubes are long. When fouling occurs, the STHE must be shut down for cleaning and/or part replacement interrupting production of polymer and adding cost. Similar to a STHE, a PHE can be subject to that same disadvantages and limitations as well (e.g., fouling). While U.S. Pat. No. 9,708,428 discloses a spiral heat exchanger, it is disclosed for use in polymerizing a polymer.
Thus, there is a need in the art for new and improved polymerization processes where devolatilization of the polymer, particularly preheating of polymer solution, can be achieved without a high pressure drop, large temperature gradient and polymer fouling. The present disclosure provides polymerization processes where devolatilization of the polymer is achieved by preheating the polymer in spiral heat exchangers with increased heat transfer capabilities, smaller footprints, lower temperature gradients and lower pressure drops.